1. Field of the Invention
The invention relates to a computed tomography method which includes the following steps:
generating a conical radiation beam which traverses an examination zone or an object present therein, PA0 generating a relative motion, including a rotation about an axis of rotation, between the radiation beam and the examination zone or the object, PA0 acquiring, during the relative motion, measuring data which is dependent on the intensity in the radiation beam to the other side of the examination zone, PA0 reconstructing the spatial distribution of the absorption within the examination zone from the measuring data acquired by the detector unit. PA0 a) defining at least one first and one second sub-volume within the overall volume traversed by the radiation beam, PA0 b) reconstructing the spatial distribution of the absorption within the first sub-volume by means of a first reconstruction algorithm, PA0 c) reconstructing the spatial distribution of the absorption within the second sub-volume by means of a second reconstruction algorithm which deviates from the first reconstruction algorithm.
The invention also relates to a computed tomography apparatus for carrying out the above method.
2. Description of Related Art
A "conical" beam is to be understood to mean a beam of finite dimensions in two mutually perpendicular directions and is detected by a detector unit which is suitable for spatially resolved measurement in these two directions of the intensity of the beam which has been attenuated in the examination zone. A method of this kind is known from a publication by L. A. Feldkamp et al. "Practical Cone Beam Algorithms", Journal of Optical Soc. Am. A, Vol. 1, No. 6/pp. 612-619, 1984.
It is a fundamental drawback of CT methods (CT=computed tomography) utilizing conical radiation beams that some voxels (volume elements) in the examination zone are only temporarily exposed to the radiation during the relative motion between the radiation source and the examination zone and that the absorption in these voxels cannot be reconstructed from the measuring data acquired by the detector unit. The part of the examination zone in which the spatial absorption distribution can be reconstructed, therefore, is always smaller than the part exposed to the radiation.
The known method utilizes a reconstruction algorithm for reconstructing the absorption within the rotationally symmetrical zone which is exposed to radiation during the entire relative motion and is shaped like a disc, the reconstruction in practice being limited to a plane slice within this zone. The known method is based on a circular relative motion.
However, there are also CT methods which involve a helical relative motion. In the case of such methods the absorption is not reconstructed in the voxels which are present in the radiation beam at the beginning or at the end of the relative motion.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.